After printed circuit boards have been manufactured, and before they can be used or placed into assembled products, they must be tested to verify that all required electrical connections have been properly completed and that all necessary electrical components have been attached or mounted to the board in proper position and with proper orientation.
Other reasons for testing are to determine and verify whether the proper components have been used and whether they are of the proper value. It is also necessary to determine whether each component performs properly (i.e., in accordance with the specification). Some electrical components also may require adjustment after installation.
Most testers utilize a "bed-of-nails" design, which includes a probe surface having plural (thousands) of sockets that are interconnected to test equipment, such as a computer with the appropriate software. Test probes are insertable in these sockets and protrude upwardly from the probe surface. These probes are configured to match the input/output connection points of the electronic components, such as integrated circuits, resident on the printed circuit board (PCB) being tested. Further, the probes are biased upwardly such that, to ensure proper alignment, a card must be placed over the probes and sufficient downward force must be provided to the PCB such that proper electrical connection is made between the inputs/outputs of the electronic components and the test equipment, via the biased test probes.
Fixturing systems have been developed for purposes of handling printed circuit boards for testing. The most common of such fixturing systems is a vacuum fixture. There are many disadvantages associated with vacuum fixturing. In vacuum fixturing, atmospheric pressure acts directly on a PCB with a vacuum beneath it, forcing the board against spring loaded testing probes. Problems arise from the need to maintain a seal around and across the board. Maintaining a vacuum seal in an automated environment is even more troublesome. Warped printed circuit boards are commonly encountered and require a separate effort or effect to push and seat them in the fixture gasketing material. PCBs with holes or apertures generally complicate vacuum fixturing techniques because of the difficulty associated with maintaining a proper seal. Also, probe density is limited by atmospheric pressure. The seals and gasketing required also involve much periodic maintenance, and contaminants and other foreign matter may be aspirated by the fixture due to the vacuum. Furthermore, vacuum fixtures generally do not provide sufficiently forceful contact between the probes and PCBs to displace contaminants present on the board surfaces, thereby necessitating additional costs and chemical disposal issues associated with pre-cleaning the boards before testing.
In response to the aforementioned problems associated with vacuum fixturing systems, other fixturing systems have been developed, including pneumatically powered systems. The typical pneumatic fixturing system incorporates a flat plate attached to a cylinder. Pneumatic pressure is applied to the cylinder which in turn forces the plate against the printed circuit board disposed on the probes. Testing problems arise from the fact that the center of the plate receives the majority of the force applied by the cylinder. Accordingly, the periphery of the board may not sufficiently contact and be tested by the probes. This is especially true with large and/or thin PCBs. Further, such pneumatic systems are not height adjustable relative the probes and thus are unable to accommodate boards of varying thicknesses and/or component heights.
Accordingly, what is needed in the art is a printed circuit board testing system that enables rapid and evenly distributed pressing of the boards to the test probes, allows adjustability of the press height so as to accommodate differently sized boards, and is modifiable to multiple configurations so as to enable multiple modes of operation.